How do you pick which girl to buy a drink for at a bar? Girls, how do you decide to go on that second date? How do you know it’s going to last? Good looks, great personality, common interests? Think again. Come a little bit closer and take a sniff. If he smells good, he’s a keeper, if not, say goodbye. Unless, of course, you’re on the pill.

You’re kidding, right?
Humans (and mice and fish and even lizards) are able to identify good potential mates by smell. One component of the scent is an immune complex called the MHC (Major Histocompatibility Complex). All humans express multiple MHC proteins on each cell in the body. The MHC helps the immune system identify and attack invaders, ie bacteria, viruses, other bad stuff. In the entire human population there are over 200 MHC genes. What that means is that the chances that my MHC and your MHC are the same is very low.

So where does smell come in?
Humans, and especially women, can smell MHC complexes. This allows women to identify mates with MHCs unlike their own. Men with different MHCs smell good, while men with similar MHCs smell bad. Unless, of course, you’re on birth control, in which case the situation is reversed.

Why on earth???
There are two theories about why this might be:
1) To prevent inbreeding.
2) To give the offspring a better immune system and therefore a better chance of surviving.

Back in the day (we’re talking way waaaaay back in the day), when humans lived in small communities it was important to identify who was related, and not mate with them. Inbreeding is bad because it allows deletrious mutations to accumulate. Hence, now laws in most states preventing cousins from marrying.

Since there are multiple MHC genes in the human population, and each person expresses multiple MHC proteins, it’s better to have all different MHCs. Children will have different MHCs if their parents have two different sets of MHCs. Otherwise, the child could wind up with limited MHCs, and a higher chance of an infection escaping the immune system.

So why does birth control change all that?
The hormones in birth control mimic pregnancy. During pregnancy, women aren’t looking for a mate, it’s more likely that they’re looking for relatives to nest with. Therefore, men with similar MHCs (who are more likely to be related) smell better.

Um, so why do I care?
Well, if you’re on the pill, you might care because couples with similar MHCs also have a harder time conceiving. So that might not matter now, but down the road, when you want to have kids, you might have a harder time. Although all this is further complicated by a study among Icelandic couples that found that as long as you’re not too related (ie 1st or 2nd cousins) the more unrelated you are, the less children you’ll have.

THE BOTTOM LINE
Pay attention to all your senses while you’re dating and you’ll be fine. And think about the pill.